The work material types of cast iron containing lamellar graphite are standardized. They are referred to the tensile strength of a sample rod with a raw cast diameter of 30 mm. The tensile strength is influenced by the graphite formation, the amount of graphite and the matrix, which can have a pearlitic/ferritic to pearlitic structure. The structure of the matrix can be objectively influenced by the addition of alloying elements. The pearlite structure is stabilized by the elements manganese, chromium, copper and tin with increasing effectiveness. The addition of these elements to the cast iron represents a noteworthy cost factor (for example of copper or tin) and, in the case of chromium and manganese, can only take place up to certain upper limits. In particular, alloying elements such as manganese and chromium increase the danger of ledeburitic hardening so that they can only be used up to a maximum of 1 or 0.5% by weight, respectively, without having a negative action on the workability of the cast iron. In the case of the use of chromium, it is to be observed that the chromium content, even in the case of repeated remelting, is not broken down. This also applies to copper. This state of affairs also places limits on the alloying of copper although a pearlite stabilization without the additional danger of ledeburitic hardening becomes possible. Furthermore, only relatively expensive, very pure copper can be used in order to exclude a damaging action of trace elements which can be present in the copper. Thus, the production of cast iron workpiece with the use of chromium- and copper-alloyed circulation material involves considerable disadvantages.
A very effective pearlite stabilization is achieved with tin but the costs are significant. With this element, there is achieved a completely pearlitic structure in all regions of the cast piece. However, a further addition of tin to cast iron which already has a completely pearlitic structure does not further increase its tensile strength.
Besides the above-mentioned elements, it is also known to introduce nitrogen into cast iron as alloying element in order to improve the strength and workability of the work material. Thus, in Federal Republic of Germany Patent Specification No. 16 08 409, there is disclosed the introduction of nitrogen into a cast iron alloy with a conventional base composition in order to avoid a structural change from pearlite to ferrite at high working temperatures. The introduction of the nitrogen thereby takes place by the addition of a nitrogen compound which is pressed into tablets with an inoculant. In this way, 0.009 to 0.018% of nitrogen is admittedly alloyed into the cast iron but, in addition, the components of the inoculant are also introduced. No statements are made regarding the manner and in what amounts the nitrogen compounds with inoculants are added to the cast iron so that a use of the process is made difficult or a dependable adjustment of the nitrogen content in the cast iron is not ensured.
Federal Republic of Germany Patent Specification No. 24 02 945 describes a high-strength cast iron containing spheroidal graphite and a process for the production thereof. The introduction of nitrogen to give contents of 0.0035 to 0.02% by weight of nitrogen takes place by means of nitrogen-containing alloys, such as Fe-Mn-N and Fe-Cr-N, or with hexamine.
In the case of the use of nitrogen-containing alloys, the nitrogen availability and thus the nitrogen yield is relatively widely distributed. The nitrogen yield is very low so that the alloys have to be used in relatively large amounts which, in turn, results in undesirable effects, such as the introduction of a relatively large amount of foreign metal, increased production of slag and insufficient solubility behavior of the nitrogen. Since hexamine is a high explosive (see P. Karrer, Lehrbuch der Organischen Chemie, 1963, p. 500), the use thereof is dangerous.
The solubility equilibrium for nitrogen in cast iron is influenced by the alloying elements and the temperature of the molten iron. In the temperature range of from 1400.degree. to 1550.degree. C., which is usual therefor, the solubility equilibrium for nitrogen in technical cast iron alloys lies at 40 to 60 ppm. The result of this is that all nitrogen contents which lie above the value of this solubility equilibrium are unstable. The loss of nitrogen is usually 10 ppm per 30 minutes standing time of the melt at 1480.degree. C. A subsequent correction of the nitrogen content by the addition of further nitrogen-containing alloys is no longer possible because of the increase of the manganese or chromium values involved therewith which would give rise to an undesired ledeburitic hardening of the cast iron.